The present invention relates generally to web-handling apparatus. More particularly, the invention relates to an apparatus and method for controlling tension within a web, specifically in web-handling systems wherein the web is moved continuously, intermittently, at varying speeds, or even in a reverse direction.
Many different forms of apparatus are known for handling a moving web. Often, the specific construction and means of operation of these various systems depends in part upon the characteristics of the web material with which the system is designed to operate, as well as the nature of the operations to be performed on the web.
One general class of web material includes photosensitive materials suitable for the reproduction of visual images. One type of such material is disclosed in U.S. Pat. Nos. 4,440,846 and 4,399,209, which describe an imaging media for use with an imaging system. In the media, a photosensitive layer is formed comprising microcapsules containing a photosensitive composition in the internal phase. The photosensitive layer is image-wise exposed to actinic radiation, and subjected to a uniform rupturing force whereupon the microcapsules rupture and image-wise release the internal phase. An image-forming chromogenic material, such as a substantially colorless color former, is typically associated with the microcapsules. When the microcapsules rupture, the color former image-wise reacts with a developer material and produces a color image. In the embodiments described in the referenced patents, the microcapsules are typically ruptured by passing imagewise exposed imaging sheets through the nip between a pair of parallel calender rolls.
This photosensitive media may exist in a form which comprises two layers. The microcapsules are carried on a continuous web referred to as a donor web. The developer composition is coated onto a second, separate substrate layer, typically formed as separate sheet, with each sheet referred to as a receiver sheet. The donor web is subjected to the actinic radiation, and the exposed microcapsule layer is then brought into contact with the developer layer of the receiver sheets. As the sheet and web are subjected to pressure, the finished image is formed in the receiver sheet.
Alternatively, the developer composition may be coated directly onto the web material. In such a case, receiver sheets are not required, and the finished images are produced directly in the web. Typically, the individual images will then require cutting from the web material.
In processing a web of the above-described photosensitive media, or any other photosensitive material, it is necessary to withdraw the material from a supply roll, move the material through an exposure station, and finally through a developer station. Moreover, it is necessary that the various operations on the web be properly registered to ensure proper positioning of the finished image on the output sheets. This in turn requires that web tension be carefully controlled, so that web positioning at the various stations can be accurately determined.
Many various means for controlling tension within a movable web are known. However, when operating upon a photosensitive material, a number of circumstances exist which make proper tension control relatively difficult. For example, exposure is most typically performed with the web stationary at the exposure station. The web is moved between exposures to advance the web, and thus frequent starting and stopping of the web occurs. The various accelerations and decelerations placed upon the web as a result causes significant tension variation within the web. Further, where a single exposure is to be made, a certain amount of web will normally be wasted due to the distance between the exposure and developer stations. To use this material, web motion in a reverse direction may be required. This too can produce substantial tension variation within a web, and also requires winding of the web back onto the supply roll.
One further problem in handling the microencapsulated web described herein is that contact with the web on the side carrying the capsules should be minimized. Otherwise, the capsule layer of the moving web could be damaged, which will in turn produce defective finished images.
What is needed, therefore, is an apparatus and method for controlling the tension within a movable web. Such apparatus and method should be capable of carrying out this function despite frequent accelerations and decelerations within the web, as well as complete reversal of web movement. Further, such an apparatus and method should be capable of compensating for changes in supply roll diameter and other changes in operating conditions. Moreover, the apparatus should operate in contact with only one side of the web.